1. Field of the Invention
The present invention relates to a method for controlling a magnetic disk memory which comprises a plurality of magnetic disks seated on a common rotational axis, the magnetic disks respectively having concentrically arranged tracks, and a plurality of magnetic heads respectively assigned to a disk surface and arranged in a radially reciprocating fashion via a common positioning device, whereby the respective tracks arranged above or below one another and all belonging to the disk surfaces are combined to form a respective cylinder and all magnetic heads are positionable in common to respectively one of the cylinders and whereby each track is subdivided into individual sectors respectively comprising a header label and a data field and an alternate sector is assigned to a plurality of sectors and serves as a useful sector in case of a defective sector occurring along the track.
2. Description of the Prior Art
It is known in magnetic disk memories to divide the concentrically arranged tracks into a plurality of sectors and to provide an alternate sector per track which can be accessed as needed. In the method known from the IBM Technical Disclosure Bulletin, Vol. 25, No. 10, March 1983, pp. 5193-5194, for example, the assignment of the alternate sectors occurs such that a sector recognized as defective and the following sectors are respectively shifted by one sector toward the end of the track, are renumbered, rewritten and appropriately identified, whereby the last useful sector is shifted into the immediately adjoining alternate sector within the same track. Since only a single alternate sector is provided per track, this displacement method can only be employed in the case of a single defective sector within the track. Given a second defective sector within the track, there is then only the possibility of externally transferring the second defective sector directly into a free, unused alternate sector in another track. This, however, necessarily causes a considerable loss of performance of the disk memory system since, due to the fact that a skip at the second defective sector from the original track onto the alternate sector in another track and, under given conditions, a return onto the original track subsequently occurs, an additional revolution of the disk is required,this involving a considerable loss of time.